I was talking with one of the instructors at a local BMWCCA driving school this past weekend (he raced an E30 M3 and is now getting ready to build an E36 race car). He told me that there is a kit, made by BMW, that would reinforce the rear shock towers to support a true coilover kit.

Has anyone else ever heard of this or done this? Would this be something that should/should not be done to a car that is also driven on the street? I just took a quik look at the SCCA auto-x rules (posted on my website if you want to take a look) and it seems like doing this type of suspension mod is in the "grey" area for both SP and SM, what do you guys think?

We have done it on our race car, but only after reinforcing the rear shock towers by tying them into the roll cage and adding additional metal to the area.

I can't imagine that anything other than welding additional metal and attaching the cage to the structure is going to give the addtional support needed. Did he mention what the "kit" consisted of? I'm guessing its a bit more involved than just an addtional plate to bolt on. i.e. not an option for the street, and probably not legal for auto-x.

Besides, what are you trying to accomplish with a rear coilover than you can't accomplish with an adjustable height spring mount in the stock location?

He said it was actual BMW motorsport part numbers, but I don't have any other data points other than that. But you are probably right that it would be a bit "involved."

I most likely would not change my current GC setup, but I was curious about this and wanted more info on it. After all, my car may turn into a "true" track car some day :cool: !!

It takes you out of even SM because the suspension pickup points (i.e the general geometry) is changed. You would be in Modified, I beleive.

According to GC (who do sell a real coilover setup for the rear) you have to tie in the cage in order to use the real coilovers.

Very few BMW's around the country are running a coilover in the rear. Even on a full track car, there's really just not that much of an advantage...and losing the rear suspension mounting point will instantly throw you into modified class in BMW CCA Club Racing, which means spending a lot more $$ to get the rest of the car competitive.

Originally posted by 95 MPWR
It takes you out of even SM because the suspension pickup points (i.e the general geometry) is changed. You would be in Modified, I beleive.

Ummm... the suspension pickup points would be the same assuming that the shock stays in it's current position with the only change being that there is now a spring around it.

The shock still mounts in the same way. No change in pickup points.

The issue would be reinforcing the area.... although if you tied the cage into that and 'reinforced' it with the 6x6 plate, then it would most likely be completely legal for SM.

FWIW, I ran rear coil-overs on my SM mustang and they were completely legal as I didn't use any holes other than what was supplied by Papa Ford.

John.

If one would convert to coilover shocks on the rear, would you retain, or totally remove the factory rear springs?
Also what would be the comparative percentage differences in front to rear spring rates be, in either case?
m.

I know of several cars that have been converted to full coilover suspensions, including the ex-PTG E36M3 and the BMW CCA Mod club racers that the shop that does race prep on my car maintains.

All of these cars have been extensively modified, with large pieces of the unibody removed and replaced with pretty much a tube frame from the passenger compartment back. This tube frame is fully integrated into the cage structure, and also protects the fuel cell. The suspensions of these cars have also been radically modified - the subframes have been discarded, and an entirely custom new suspension has been designed and built.

All in all, pretty much as far as you can get from a simple bolt-on reinforcement plate. Since the entire suspension geometry has been changed, this wouldn't be legal for Street Mod or Street Prepared Solo2 - you'd be in either Mod or Prepared.

Definitely illegal for Street Prepared - check out section 14.8.A in the rules. "Springs must be of the same type as the original (coil, leaf, torsion bar, etc.) and except as noted herein, must use the original spring attachment points." The exceptions in this section have to do with changing to adjustable spring perches. Not even in any of the "gray" areas - stone cold illegal.

Jim

Originally posted by mmark.
If one would convert to coilover shocks on the rear, would you retain, or totally remove the factory rear springs?

Umm...it wouldn't be a coilover if you were still using the factory spring position.

One of the advantages is that you get to use a softer rear spring. Why is this an advantage? you can use a lighter spring that has more range before coilbind, due to thinner spring wire.

But, to most of us, it's not worth it. :)

Originally posted by kitwetzler
One of the advantages is that you get to use a softer rear spring. Why is this an advantage? you can use a lighter spring that has more range before coilbind, due to thinner spring wire.

You can also run a much longer rear spring (since you have a much larger range of adjustment on a thread shock body), which helps alleviate coilbind as well.